Smashing Black Holes Make Gravitational Waves, Again

The LIGO gravitational
wave detector has witnessed two small black holes collide and merge as
one, confirming that the original gravitational wave discovery was no
fluke.

Only months after the historic discovery of gravitational waves,
physicists have done it again! LIGO has detected ANOTHER black hole
collision and confirmed the first gravitational wave detection wasn't a
one-off.

On Dec. 26, the extremely faint spacetime ripples
washed through our planet and the Laser Interferometer
Gravitational-wave Observatory, or LIGO, was listening. The US-based
detector recorded the distinctive gravitational wave "chirp", meaning
that, once again, we were witness to a collision of cataclysmic
proportions.

These ripples in spacetime were first theorized by
Albert Einstein over 100 years ago when he formulated his theory of
general relativity , but it's only now that humanity has the tools to
actually prove they exist. And this most recent detection is a firm
confirmation that, once again, Einstein was right.

In
a galaxy, some 1.4 billion light-years away, two small black holes got
stuck in an inescapable gravitational spiral. Their fate was sealed;
they fell closer and closer until they rapidly span around one another,
colliding and merging as one. Like the first historic detection of
gravitational waves in September, this most recent signal originated
from a black hole merger, an event that shines a previously unattainable
light on one of the most violent collisions in the universe.

Interestingly, this most recent event, called GW151226, included
black holes that were much smaller, the pair "weighed in" at only 14 and
8 times the mass of the sun -- the September event, called GW150914,
consisted of two merging black holes of 29 and 36 times the mass of the
sun. During both events, as the black holes pairs rapidly spiraled in,
they warped spacetime, generating gravitational waves. During the
collisions, "new", more massive black holes emerged from the collision
and information of these mergers were encoded in their gravitational
wave signals that LIGO could detect and decipher.

The December
event spawned a new black hole of 21 solar masses, but during the
collision, one whole solar mass was converted from matter into energy,
blasting powerful gravitational waves across the intergalactic expanse.
"It
is very significant that these black holes were much less massive than
those observed in the first detection," said Gabriela Gonzalez, LIGO
Scientific Collaboration (LSC) spokesperson in a statement. "Because of
their lighter masses compared to the first detection, they spent more
time -- about one second -- in the sensitive band of the detectors. It
is a promising start to mapping the populations of black holes in our
universe."

LIGO consists of two "L"-shaped detectors located in
Louisiana and Washington, a little under 2,000 miles apart. Each
building consists of two perpendicular 2.5 mile-long tunnels that guide
extremely sensitive laser interferometers. The project, funded by the
National Science Foundation and conceived and managed by Clatech and MIT
physicists, underwent a sensitivity upgrade last year, allowing
"Advanced LIGO" to detect the very slight spacetime wiggle as the
gravitational waves washed over us.
And "slight" is an
understatement. Advanced LIGO can detect the slightest warping, to a
precision of 10,000 times smaller than the width of a proton.

The
second discovery "has truly put the 'O' for Observatory in LIGO," said
Caltech's Albert Lazzarini, deputy director of the LIGO Laboratory.
"With detections of two strong events in the four months of our first
observing run, we can begin to make predictions about how often we might
be hearing gravitational waves in the future. LIGO is bringing us a new
way to observe some of the darkest yet most energetic events in our
universe."
For
the first time, humanity has been given a glimpse of the "dark"
universe, a domain that has, until now, remained invisible to us. The
collision of two black holes wouldn't necessarily generate any emissions
in the electomagnetic spectrum (i.e. light), so traditional astronomy,
for the most part, cannot witness these events. But LIGO is "feeling"
the motion of spacetime, waiting for when these violent collisions do
occur.

"With the advent of Advanced LIGO, we anticipated
researchers would eventually succeed at detecting unexpected phenomena,
but these two detections thus far have surpassed our expectations," said
NSF Director France A. Córdova.

But we are scraping the surface
of gravitational wave detections. Only this time last year,
gravitational waves were a confounding phenomenon that existed in theory
but had no direct observational evidence. Now Advanced LIGO has become
sensitive enough to detect the most powerful gravitational events in the
universe. As it becomes even more sensitive, and as even more
gravitational wave detectors around the globe go online, it's hard to
predict what else we'll find going "bump" in the gravitational night.